[ntp:questions] serialPPS+gpsd+ntpd large offset & jitter

Rob nomail at example.com
Wed Oct 20 17:24:44 UTC 2010

Miroslav Lichvar <mlichvar at redhat.com> wrote:
> However, in my experience the NMEA sources tend to be quite bad. If
> you are not planning to run without PPS or would rather use an NTP
> source as a backup, remove the line and use gpsd just for
> the PPS source.

That is very true!  I have seen NMEA sources that jitter so much that
it is impossible to keep the clock within half a second of offset, and
thus will make it very difficult to use it for absolute time info for
the PPS source.

I even observed receivers that have a big offset between the NMEA time
and real time, that changes every time you restart the receiver.  When
you have finally tweaked the fudge in ntp.conf so that the offset of
the NMEA (while synced to PPS) is within 20ms or so, the next time you
boot everything it again is 450ms or more.   Terrible.

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